Pulse generators such as pacemakers or defibrillators implanted in the body for electrical cardioversion or pacing of the heart are well known. More specifically, devices implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life-threatening arrhythmias, or to stimulate contraction (pacing) of the heart, where electrical energy is applied to the heart via electrodes to return the heart to normal rhythm. Devices have also been used to sense near the sinal node in the atrium of the heart and to deliver pacing pulses to the atrium. An electrode in the atrium positioned near the sinus node of the heart senses the electrical signals that trigger the heartbeat. The electrode detects abnormally slow (bradycardia) or abnormally fast (tachycardia) heartbeats. In response to the sensed bradycardia or tachycardia condition, a pulse generator or pacemaker produces corrective pulses or signals and delivers them to alleviate the condition.
The sick sinus syndrome and symptomatic AV block constitute the major reasons for insertion of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by electrodes implanted directly onto the ventricular epicardium. Transvenous pacing may be temporary or permanent. In temporary transvenous pacing, an electrode lead is introduced into a peripheral vein and fluoroscopically positioned against the endocardium of the right atrium or right ventricle. The proximal electrodes are connected to an external cardiac pacemaker which has an adjustable rate and milliamperage control. Temporary transvenous pacing is utilized (1) prior to the insertion of a permanent pacing system and (2) in situations in which the indication for pacing is judged to be reversible (drug-induced AV block or bradycardia) or possibly irreversible and progressive (AV and bundle branch blocks associated with myocardial infarction). Permanent transvenous pacing is performed under sterile surgical conditions. An electrode lead is generally positioned in the right ventricle or in the right atrium through a subclavian vein, and the proximal electrode terminals are attached to a pacemaker or defibrillator which is implanted subcutaneously. Another sense electrode may be positioned within the atrium of the heart near the sinus node.
The implanted pulse generator typically has a number of electronic components held within a sturdy case which both provides a hermetic and a protective enclosure for the electronics. The typical casing is constructed of titanium or stainless steel and has a wall thickness which is substantial enough to be relatively rigid providing structural integrity for the electronics as well. One type of case is a clamshell construction having two shallow concave case halves which define a perimeter parting line when mated together. The parting line is laser welded to close and seal the clamshell case. Another type of case is a deep drawn pocket type enclosure having an open end. The end is covered by a plate or end cap and again laser welded to the pocket enclosure.
Because the case must provide both hermeticity and structural integrity for the device, a typical case accounts for about 5-10% of the volume of the device. Because the material is typically metallic and has a substantial wall thickness, the case also typically accounts for about 15-25% of the weight of the implant device. As today's electronics become smaller, these percentages have increased and will continue to do so accordingly.
Size and weight are two important concerns for implantable devices. It is desirable to reduce the weight and the size or volume of such devices wherever possible. The need for the case to provide structural integrity to the implantable device is what drives the minimum wall thickness of the case material. Therefore, if this requirement were eliminated, the size and weight of the case, and hence the implantable device, could be significantly reduced.
There is a real need for an implantable medical device such as a pacemaker or defibrillator having an ultra-thin hermetic case. A device equipped with such a thin enclosure would be of reduced size or volume. In addition, a device having an ultra-thin hermetic enclosure would be lighter in weight than a conventional implantable medical device.